New synthesis strategies allowing to obtain materials of well-defined porosity in a very wide range, from microporous materials to macroporous materials to hierarchical porosity materials, i.e. having pores of several sizes, have known a very large development within the scientific community since the mid-90s (G. J. de A. A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev., 2002, 102, 4093). In particular, considerable work has been done on the development of materials having a porosity of zeolitic nature and a mesoporosity so as to simultaneously benefit from the catalytic properties specific to zeolites and from the catalytic and especially the textural properties of the mesoporous phase.
A technique that is commonly used to generate materials having such biporosity consists in directly creating mesopores within zeolite crystals by subjecting the zeolite to a steam-hydrothermal treatment, also referred to as steaming. Under the effect of this treatment, the mobility of the tetrahedric atoms that make up the framework of the zeolite is increased to such an extent that some of these atoms are extracted from the network, which causes formation of amorphous zones that can be removed to give way to mesoporous cavities (A. H. Jansen, A. J. Koster, K. P. de Jong, J. Phys. Chem. B, 2002, 106, 11905). The formation of such cavities can also be obtained by subjecting the zeolite to an acid treatment (H. Ajot, J. F. Joly, J. Lynch, F. Raatz, P. Caullet, Stud. Surf. Sci. Catal., 1991, 62, 583). These methods however have the drawback of making part of the zeolite partly amorphous and of modifying the properties thereof through variation of the chemical composition. In any case, the mesoporosity thus introduced allows to eliminate or at least to limit diffusion limitation problems encountered in microporous materials, mesopores having much greater diffusion factors than micropores and thus allowing access to the active sites of the zeolites (P. B. Weisz, Chemtech, 1973, 3, 498).
More recently, much work has been done on the elaboration of mixed mesostructured/zeolite materials, mesostructured materials affording the additional advantage of a perfectly organized and calibrated porosity in the mesopore range.
It can be briefly reminded here that mesostructured materials are conventionally obtained via synthesis methods referred to as soft chemistry methods that consist in bringing together, in aqueous solution or in polar solvents, inorganic precursors with structuring agents, generally molecular or macromolecular surfactants, ionic or neutral. Control of electrostatic interactions or of interactions through hydrogen bonds between the inorganic precursors and the structuring agent jointly linked with hydrolysis/condensation reactions of the inorganic precursor leads to a cooperative assembly of the organic and inorganic phases generating micellar aggregates of surfactants of uniform size controlled within an inorganic matrix. Clearance of the porosity is then obtained by surfactant elimination, which is conventionally carried out by means of chemical extraction processes or by thermal treatment. Depending on the nature of the inorganic precursors and of the structuring agent used, and on the operating conditions applied, several families of mesostructured materials have been developed, such as the M41S family obtained using quaternary ammonium salts as the structuring agent (J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 27, 10834) or the SBA family obtained using three-block copolymers as the structuring agent (D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredickson, B. F. Chmelka, G. D. Stucky, Science, 1998, 279, 548).
Several synthesis techniques allowing elaboration of such mixed mesostructured/zeolite materials have thus been listed in the open literature. A first synthesis technique consists in synthesizing in a first stage a mesostructured aluminosilicate material according to the conventional methods described above, then, in a second stage, in impregnating this material with a structuring agent commonly used in the synthesis of zeolitic materials. A suitable hydrothermal treatment leads to a zeolitization of the amorphous walls of the initial mesostructured aluminosilicate (K. R. Koletstra, H. van Bekkum, J. C. Jansen, Chem. Commun., 1997, 2281; D. T. On, S. Kaliaguine, Angew. Chem. Int. Ed., 2001, 40, 3248; D. T. On, D. Lutic, S. Kaliaguine, Micropor. Mesopor. Mater., 2001, 44, 435; M. J. Verhoef, P. J. Kooyman, J. C. van der Waal, M. S. Rigutto, J. A. Peters, H. van Bekkum, Chem. Mater., 2001, 13, 683). A second synthesis technique consists in bringing together a colloidal solution of zeolite seeds and a surfactant commonly used to create a mesostructuration of the final material. The basic idea here is to simultaneously generate the elaboration of an inorganic matrix of organized mesoporosity and the growth, within this matrix, of zeolite seeds so as to ideally obtain a mesostructured aluminosilicate material with crystallized walls (Z. T. Zhang, Y. Han, F. S. Xiao, S. L. Oiu, L. Zhu, R. W. Wang, Y. Yu, Z. Zhang, B. S. Zou, Y. Q. Wang, H. P. Sun, D. Y. Zhao, Y. Wei, J. Am. Chem. Soc., 2001, 123, 5014). A variant of these two techniques consists in starting from a mixture of aluminium and silicon precursors in the presence of two structuring agents, one likely to generate a zeolitic system and the other likely to generate a mesostructuration. This solution is then subjected to two crystallization stages under variable hydrothermal treatment conditions, the first stage leading to the formation of the mesoporous structure of organized porosity and the second stage leading to the zeolitization of the amorphous walls (A. Karlsson, M. Stocker, R. Schmidt, Micropor. Mesopor. Mater., 1999, 27, 181; A. Karlsson, M. Stocker, R. Schmidt, Stud. Surf. Sci. Catal., 2000, 129, 99). All of these synthesis methods have the drawback of damaging the mesostructure and thus to lose the advantages thereof in cases where growth of the zeolite seeds or zeolitization of the walls is not perfectly controlled, which makes these techniques delicate to implement.
It can be noted that it is also possible to directly elaborate composite mesostructured/zeolite materials so as to take advantage of the catalytic properties specific to each one of these phases. This can be done through thermal treatment of a mixture of a zeolite seed solution and of a mesostructured aluminosilicate seed solution (P. Prokesova, S. Mintova, J. Cejka, T. Bein, Micropor. Mesopor. Mater., 2003, 64, 165) or through growth of a zeolite layer at the surface of a presynthesized mesostructured aluminosilicate (D. T. On, S. Kaliaguine, Angew. Chem. Int. Ed., 2002, 41, 1036).
To the exclusion of the mesoporous zeolitic materials obtained through post-treatment of a zeolite, we note that, from an experimental point of view, all these materials are obtained by direct precipitation of inorganic precursors in the presence or not of structuring agents within an aqueous solution or in polar solvents, this stage being in most cases followed by one or more ripening stages in an autoclave. The elementary particles usually obtained exhibit no regular shape and they are generally characterized by a size ranging between 200 and 500 nm.
Work has also been done on the elaboration of materials exhibiting both microporosity and macroporosity. By way of example, one of the most commonly used synthesis methods consists in using polystyrene balls as the macroporosity-generating element and in creating around these balls a zeolitic network (G. S. Zhu, S. L. Qiu, F. F. Gao, D. S. Li, Y. F. Li, R. W. Wang, B. Gao, B. S. Li, Y. H. Guo, R. R. Xu, Z. Liu, O. Terasaki, J. Mater. Chem., 2001, 11, 6, 1687).